When ships are hauled into docks for repairing, the surface of the hull is cleaned with pressurized water first to remove salt and deposits, then blasted at corroded portions and finally coated with paint. After these hull surface treating procedures have been completed in series, the ship is hauled out of the dock. The operation of hauling the ship into and out of the dock and the surface treating operation are the chief work at the dock, which must therefore be equipped with apparatus for performing these operations. Self-running surface treating trucks provided on opposite side walls of the dock are usually used for the surface treating operation, while ships are hauled into and out of the dock usually by a rope. With such a tow apparatus, however, the rope must be tied to and removed from the ship for docking manually by many workers, so that the work temporarily reaches a peak and involves problems in respect of safety. Furthermore, docks of larger scale receive larger ships which require a rope of increased diameter to result in a higher peak in the amount of work. To meet the situation, it appears useful to provide on opposite side walls of the dock self-running docking trucks for hauling the ship into and out of the dock. However, since the surface treating operation and the hauling operation are conducted not concurrently but, so to speak, alternatively, it is uneconomical to provide two types of self-running trucks separately, while there is the likelihood that one truck will become an obstacle when the other truck is used. Although it may be possible to use a single truck for both surface treatment and docking, the truck, which needs to have a great weight for hauling the ship into and out of the dock, must be driven in its entirety also for surface treatment. Generally the truck is run more frequently for surface treatment than for hauling, so that the travel of the heavey truck in its entirety for surface treatment entails an increased operating cost and is not economical.